I think this highlights how little Trump cares about any form of diplomacy or taking advice; and how few people will be prepared to stop him.
Ambassadors in the US are political appointments: they don't work in the dept of foreign affairs for years, training, they get the appointments for being some kind of political ally like a reward. Then I guess the civil servants around them do most of the work. In the U.K., being an ambassador requires years and years of work and understanding. Trump either didn't know that difference, or had no respect for it.
I'm really concerned there might be an 'enemy of my enemy is my friend' approach from a minority of leave supporters thinking the UK should give in to some of Trumps demands. He has so little respect for anyone else, the implications in the long run are frightening.
He's already said he thinking his son in law could help bring about peace between Israel and Palestine, presumably because he's Jewish (he certainly has no other qualification). It's all quite terrifying, and I don't know why more isn't being done to slap him down. America is powerful, but this really is something the rest of the world needs to unite behind.